In computed tomography, a patient's or subject's cross-section of interest is successively scanned from a number of directions by an x-radiation source to direct X-rays through the cross-section of interest. One or more detectors positioned on an opposite side of the patient obtain intensity readings of the x-radiation after it has passed through the patient. If enough intensity measurements from different directions are obtained, these intensity readings can be utilized to reconstruct a density mapping of the patient's cross-section.
Computed tomography reconstruction techniques are derived from mathematical reconstruction algorithms utilizing the fact that each radiation intensity reading corresponds to a line integral of an attenuation function taken through the patient from the source to the position the intensity is sensed. These reconstruction algorithms allocate this attenuation along the path the radiation takes in traversing the patient in a process known as back projection.
Fourth generation computed tomography designs include a circular array of stationary detectors and a moving x-radiation source. The fourth generation array of detectors typically surrounds a patient aperture which defines a patient scanning plane. An X-ray source then radiates the plane from a number of spaced locations. This scanning is typically achieved by orbiting an X-ray tube about the patient and detecting X-ray intensities of radiation passing through the patient.
In early and current third generation computed tomography scanner designs the detectors also move as the patient is scanned. In orbital CT designs, for example, the X-ray source and an arc of detectors orbit in unison about the patient.
A common need in all commercial computed tomography scanners known to applicants is a motive force for moving at least the X-ray source and in some designs both the source and detectors. Heretofore, this motive force was often provided by a motor coupled to a support for the X-ray tube via either a belt or gear linkage.
Such a motor must be capable of applying a large torque to a fairly large X-ray frame to accelerate the frame to a high constant rate of rotation in a short time. As an indication of the speeds which are reached, sufficient data to produce an image can be collected in as little as one second. In a fourth generation CT scanner designated the Synerview 1200 which is commercially available from Picker International Inc. of Cleveland, Ohio, the rotating CT apparatus defines an aperture of sufficient diameter to allow a patient's torso to be inserted for scanning. Since the rotational inertia of this apparatus increases with distance from the axis of rotation, provision of this full body scanning capability results in rotating apparatus (including an X-ray tube) having a large inertia requiring high torques.
Belt linkages have an advantage in cost which is offset by their inability to provide sufficient torque for high speed computed tomography scanning. To provide sufficient torques to accelerate the Synerview 1200 scanner a motor having magnets mounted to the rotating frame and field coils mounted to the stationary gantry are used. This induction motor is more expensive than belt linkages but provides higher torques needed to rapidly accelerate the X-ray tube. The cost disadvantage of the induction motor is at least partly attributable to a complex mounting scheme for the motor windings, scanner bearing and motor magnets.